The first part of your question is: Is Islam a religion?
The answer is: Islam is not a religion in the usual sense. Religion is defined as "a set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs."
Islam is much more than a religion as defined above. Islam is a way of life based on the guidance of God given in the books of God and the example of the prophets of God. Islam literally means "Submission" (to the One True God).
It was the religion of all the Prophets of God including the first man and Prophet Adam, the Patriarch Abraham, Moses, Jesus and last Prophet Muhammad, (peace be upon them all).
Usually religion is understood to mean a private relationship between man and God. Islam is entirely different from this kind of religion, because it is comprehensive in the sense that it involves all the aspects of life as lived by humans.
It is those who have misunderstood the nature of Islam who accuse it of being more radical than spiritual. Some who wish to apply the strategy of "divide-and-rule" to Muslim lands would like to split up the world Muslim community into sects following different "brands" of Islam.
They would speak of political Islam, fundamentalist Islam, leftist Islam, rightist Islam and so on. But such divisions have little to do with the reality of Islam. To Muslims of the world, there is only one Islam; and it is the Islam of the Quran, which Muslims believe is the word of God, and Sunnah (the Example) of Prophet Muhammad (peace be upon him).
It is the Islam believed in and followed by all the great Companions of the Prophet and his Successors and by generation after generation of Muslims the world over.
It is true that Islam covers all aspects of life, such as politics, economics and culture, as it is a divinely revealed religion that gives norms and principles that are meant to govern human life in its varied aspects.
It is for this reason the misunderstanding of Islam being political or economical arises. But the teachings of religion are applicable not only to the private sphere of human life.
For instance, religious values, such as truthfulness, altruism and equality and fraternity of all humans apply to all spheres of human activities. And can we bypass the question of equal justice to all humans in the socio-political area of human life?
In fact, Islam does not recognize any kind of separation between religion and life. It does not share the history of the separation between church and state, for everything should be dedicated to God alone.
A Muslim is required to submit himself completely and without reservation to the will of God in all his affairs.
God Almighty in the noble Quran orders Muslims to declare:
(Say, ‘My prayers and sacrifice, my life and death, are all for God, Lord of all the Worlds;)(Al-Anam 6:162)
Of course, there are religions that do not worry about the practical application of justice to the affairs of men, and promote a spirituality that is alienated from the daily lives of people.
They leave the political and economic realms entirely to the visions of human rulers or politicians who sometimes follow Machiavellian strategies.
Islam teaches that Muslims should always strive to see that justice is done at all costs. For Muslims, justice is a very high value. And it is for this reason that Muslims in different parts of the world are seen to be fighting for justice; and this is often misunderstood as extremism.
The second question is whether Islam is a culture.
From a sociological point of view, "culture" may be defined as the complex of the beliefs, morals, practices, objects and life style common to the members of a group or society.
In this sense, Islam has common cultural elements. For instance, Muslims all over the world pray five times a day as a congregation in a mosque. They do not consume alcohol or drugs; and for a whole month they fast during day time, etc.
Since Islam covers political and economic aspects of human life also, it cannot simply be called a culture. So, the answer to the question is that Islam is not just a culture; though it possesses and promotes a distinctive culture characteristics of its own.
Your next question is: Is Islam a nationality?
Nationality is usually a legal relationship between an individual and a state. The term normally signifies a geographical boundary.
Islam is transnational and its adherents are spread over different countries in different continents; they are nationals of several countries.
It is particularly noteworthy that Islam does not encourage people to be divided on the basis of the country of birth or domicile.
It is strongly opposed to the idea of nationalism, which is a vehemently passionate affinity for one's country that makes a person hate the nationals of other countries.
Yet, Islam encourages patriotism, which is genuine love for one's country. Nationalism tends to divide humans; but Islam unites humans; it preaches One God, and one humanity.
The next question is: Is Islam an ethnicity?
In the Quran, God Almighty says what means:
(People, We created you all from a single man and a single woman, and made you into races and tribes so that you should recognize one another. In God’s eyes, the most honored of you are the ones most mindful of Him: God is all knowing, all aware.)(Al-Hujrat 49:13)
The above verse makes it extremely clear that Islam would bring the entire human race under the principle of One God as it does not give any tribe, race, or nation precedence over another.
In short, people’s ethnic identity is not all a criterion for their worth according to Islam. It should be underscored that the concept of the oneness of humanity based on the Oneness of God is central to Islam.
I hope this answers your question. Please, keep in touch.